1. Field of the Invention
The present invention relates to the control of vibrations. It is particularly, but not exclusively, concerned with controlling vibrations in an automobile, such as vibrations between the engine and its mounting.
2. Summary of the Prior Art
EP-A-0115417 and EP-A-0172700 discussed two different types of hydraulically damped mounting devices for damping vibration between two parts of a piece of machinery, e.g. a car engine and a chassis. EP-A-0115417 disclosed various “cup and boss” type of mounting devices, in which a “boss”, forming one anchor part to which one of the pieces of machinery was connected, was itself connected via a deformable (normally resilient) wall to the mouth of a “cup”, which was attached to the other piece of machinery and formed another anchor part.
The cup and the resilient wall then defined a working chamber for hydraulic fluid, which was connected to a compensation chamber by a passageway (usually elongate) which provided the damping orifice. The compensation chamber was separated from the working chamber by a rigid partition, and a flexible diaphragm was in direct contact with the liquid and, together with the partition formed a gas pocket.
In EP-A-0172700 the mounting devices disclosed were of the “bush” type. In this type of mounting device, the anchor part for one part of the vibrating machinery is in the form of a hollow sleeve with the other anchor part in the form of a rod or tube extending approximately centrally and coaxially of the sleeve. In EP-A-0172700 the tubular anchor part was connected to the sleeve by resilient walls, which defined one of the chambers in the sleeve. The chamber was connected via a passageway to a second chamber bounded at least in part by the bellows wall which was effectively freely deformable so that it could compensate for fluid movement through the passageway without itself significantly resisting that fluid movement.
Both the two types of mounting devices discussed above are passive, in the sense that they have components which are influenced by vibrations, and thus provide damping, but do not actively seek to counter those vibrations by applying opposed vibrations. In EP-A-0262544, a modification of the “cup and boss” type mounting device was proposed, in which the damping characteristics of the mount were changeable in dependence on frequency of vibration. This provided a “semi-active” mount, but still did not provide a mounting device in which there was active imposition of vibrations to counter the vibrations applied to the mounting device. However, it is known to apply such vibrations, to provide an active mount in which there is cancellation of the vibrations applied thereto. Such mounts sense the presence of steady periodic components in the vibration applied to the mount, e.g. from an automobile engine, and by appropriate manipulation develop an opposing variable force leading to cancellation of the vibrations, so that the vibrations are not transmitted to the supporting structure. In such active mounts, there must be a control relationship between the vibrations applied to the mounting device and the opposed vibrations generated by the mounting device. Existing relationships depend on prior knowledge of the characteristics of the mount, which are assumed to remain fixed. It is assumed the vibration input is predominantly of a steady periodic form e.g. a sine wave (say of frequency ω) with additional smaller random content. The aim of vibration cancellation is to input into the system an addition vibration signal which will cancel the input (i.e. a sine wave of the same frequency and amplitude but 180° out of phase). The main problem in achieving this is that generally structural components, through which the vibrations pass, tend to change both the vibration amplitude and phase. This means that what might appear to be the correct phase of a cancellation signal at one point in the structure may well be detrimental at another.
The steady periodic waveform being cancelled may be considered a superposition of a number of component sinusoidal waveforms (Fourier components) have differing frequencies amplitude and phase relative to each other. Each may be characterised by its magnitude and phase relative to some reference. Thus in the following, a particular Fourier component, of a time domain signal (say x(t)) is represented as a frequency domain vector x. Similarly the characteristics of the structure (and associated control system) through which these signals pass may be simplified by breaking it down into blocks each of which is known to have some effect on phase and amplitude of steady periodic signals. For example an accelerometer may convert a vibration expressed as a displacement amplitude into a voltage signal of a differing amplitude. The voltage signal from a perfect accelerometer will also be 180° out of phase relative to the input. Similarly an actuator should produce a force that seeks to be proportional to the displacement input voltage but in practice the force is likely to lag the input due to, e.g. inductance within the actuator mechanism. Quantitatively these effects are expressed as transfer function which give the change in phase and amplitude gain as a function of frequency. Known control systems have made use of iterative relationships following conversion of the vibrations into frequency domain phase and magnitude values.
Thus, in GB-A-2354054 use was made of vector algebra in the frequency domain and it was proposed that an iterative relationship be used in which a new vector of one iteration is derived from the old vector of the previous iteration, plus a quantity derived from historic feedback, again in vector form. A controller is then used to generate output signals for the respective iterations with the output signals being in frequency domain vector form such that the output signal of one iteration is derived from the controller output signal of the immediately previous iteration in frequency domain vector form plus a frequency domain vector quantity derived from the resultant vibration of more than one previous iteration.
The system shown diagrammatically in FIG. 1 of the accompanying drawings (which will be discussed in more detail later) can be expressed using transfer functions (G) in a block diagram as shown in FIG. 2 of the accompanying drawings. From this it is possible to write an expression for the output vibration in term of all the component effects. Each path is summed independently and the effect of the components in each path is simply the product of all the component transfer functions.
Note that, in FIG. 2, x y are the frequency domain vector representations of a Fourier component of the input and output signal, respectively. u is an output vector signal controlling the force applied to the mount by the control system.
In such a system, for a given unknown steady input x it is possible to express the relationship between y and u as follows:y=[R]•u+u0  (1)where [R] and u0 are unknowns dependent on the system transfer functions and the input x. The optimum controller output u′ leading to zero output y can then be expressed:u′=−[R]−1•u0  (2)It is possible to find a solution for u′ if two u, y data pairs exist (un−1, yn−1, un, yn)u′=un−[R]−1yn Where [R]−1 is a matrix:
                                              r            1                                                -                          r              2                                                                        r            2                                                r            1                                    ⁢          ⁢  or  ⁢          ⁢                                              r            1                                                r            2                                                            -                          r              2                                                            r            1                                  r1=(|(yn−1−yn)|)−2{(yn−1−yn)•(un−1−un)}r2=(|(yn−1−yn)|)−2{|(yn−1−yn)x(un−1−un)|}(NB “•”—dot or scalar product “x”—cross or vector product)The above is converted in an iterative control relationship that searches for the next best value of u′(n+1) based on the evidence of the last two attempts u′(n),u′(n−1).u′(n+1)=u′(n)−A[R(n,n−1)]−1•y(n)+p(n)
[R(n1 n−1)]−1 is the [R]−1 matrix based on the nth and n−1th iterations as defined above.
A is a scalar (0>A>1) defining rate of convergence and stability and p(n) is an optional small perturbation.
Hence, GB-A-2354054 proposed that the iterative control relationship defined above was applied to the active control of a mounting device, e.g. to one Fourier component, or any number or all of the Fourier components of the vibration.
Preferably, the value of A is in the range 0.1 to 0.3 and although the perturbation p(n) may be zero, it is preferably one percent or less of the size of the normal control output.
When such an arrangement was used in a hydraulically damped mounting device, it was necessary to drive the mounting device in accordance with the value of u. The mounting devices of EP-A-0115417 and EP-A-0172700 do not have means for applying such a driving force to the hydraulic fluid, since they are passive mounts as previously described, and therefore GB-A-2354054 disclosed mounts in which such a drive force could be applied.